The present invention relates to high resolution intravascular imaging and more particularly to intravascular ultrasound imaging and techniques for enhancing image quality.
In intraluminal or intravascular ultrasound (also referred to as xe2x80x9cIVUSxe2x80x9d) imaging, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. In some types of intraluminal systems, an ultrasonic unidirectional exciter/detector within a catheter probe positioned within a blood vessel is used to acquire signal data from echoes of the emitted ultrasonic energy off the interior of the blood vessel. Specifically, vectors are created by directing focused ultrasonic pressure waves radially from a transducer in a catheter and collecting echoes at the same transducer from the target area. A plurality of radial vectors from the rotated transducer comprises an image frame. A signal processor performs image processing (e.g., stabilization of a moving image, temporal filtering for blood speckle, and other image enhancement techniques) on the acquired data in order to provide a display of the corrected and filtered intravascular image on a raster-scan display monitor.
It is desirable to provide imaging over a broad range of frequencies (e.g., 5 Megahertz (MHz) to 50 MHz), especially higher ultrasonic frequencies in some applications. However, the backscatter from blood cells in such an image is a significant problem in high frequency intraluminal ultrasound imaging, since the scattering of ultrasound from blood cells is proportional to the fourth power of the frequency such that the higher the ultrasound frequency the more pronounced is the backscatter from blood. As a result, echoes from blood molecules degrade the lumen-to-vessel wall contrast, which is undesirable since there is a need to define the blood/tissue boundary in order to ascertain the degree of narrowing of the vessel and to determine the spatial extent of the plaque. Therefore, echoes in the ultrasound image due to backscatter from blood (the irregular pattern of backscatter from blood is referred to as xe2x80x9cblood specklexe2x80x9d) must be detected in order to provide an enhanced image display. Once detected, the blood speckle may be removed or suppressed to a level at which wall structures can be distinguished from blood, distinguished by providing a different display color for the blood, and/or used to better delineate the blood/tissue interface.
Various techniques have been used in intravascular ultrasound imaging for detecting blood speckle in the image. These techniques are not always effective in distinguishing between blood and tissue, because they are based on key assumptions which are not always true. Some techniques rely on the assumption that the energy scattering strength from blood is low in comparison to the scattering strength from tissue, in order to distinguish between blood and tissue. Other techniques rely on the assumption that the blood moves much faster compared to the tissue and thus has a different Doppler signal than the tissue. In reality, however, such assumptions may be violated. In particular, the energy scattering from blood can sometimes be equally as bright as the scattering from tissue, and/or blood may sometimes move with very low velocity or not be moving at all. Although generally effective, these techniques may not be so effective in situations when these assumptions are not valid.
From the above, it can be seen that alternative or supplementary methods and apparatus are needed for detecting blood speckle to allow a display of intraluminal ultrasound images to be free of or to distinctly identify blood-induced echoes.
The present invention provides methods and apparatus which detect blood speckle in an improved manner. The present invention utilizes the fact that the energy scattering strength from blood exhibits a high frequency dependency, while the scattering strength from tissue lacks a strong frequency dependency. In specific embodiments, the present invention may provide a particularly simple and useful solution for addressing the problem of blood speckle in intravascular ultrasound imaging, especially in situations where the blood may have a scattering strength similar to that of tissue and/or where the blood is moving slowly or not at all.
According to a specific embodiment, the present invention provides a method of detecting blood speckle in an intravascular ultrasound blood vessel image. The method includes the steps of illuminating an intravascular target with ultrasonic RF energy to generate ultrasonic echoes from the intravascular target, and transforming the ultrasonic echoes from the intravascular target into a received RF signal. The method also includes performing spectral analysis on at least a portion of the received RF signal to provide intensity information on the spectrum of the received RF signal. The information includes a first intensity strength at a high frequency within the spectrum and a second intensity strength at a low frequency within the spectrum. The method further includes comparing the first intensity strength and the second intensity strength, and determining that the intravascular target is tissue if the first intensity strength and the second intensity strength are approximately equal and that the intravascular target is blood if the first intensity strength is greater than the second intensity strength. This determining step takes into account strength sensitivities at the high and low frequencies. Some specific embodiments may perform spectral analysis either by complete Fourier analysis or by filtering for the high and low frequencies.
According to another specific embodiment, the present invention provides a method of detecting blood speckle in an intravascular ultrasound blood vessel image that includes the steps of illuminating an intravascular target with ultrasonic RF energy at a first frequency to generate ultrasonic echoes from said intravascular target to form a first image frame, and illuminating the intravascular target with ultrasonic RF energy at a second frequency to generate ultrasonic echoes from the intravascular target to form a second image frame. The first and second image frames are successive in time and one of the first and second frequencies is a low frequency with the other one being a high frequency. The method also includes step of subtracting the first and second image frames to obtain a subtracted image frame and the step of determining that portions of the subtracted image frame that are substantially cancelled-out are tissue and that portions of the subtracted image frame that are not cancelled-out are blood. The determining step takes into account strength sensitivities at the high and low frequencies.
According to yet another specific embodiment, the present invention provides an apparatus for an ultrasonic blood vessel imaging system. The apparatus includes a transducer having a frequency bandwidth including known and sufficiently high strength sensitivities at a first frequency and a second frequency. The transducer obtains echoes from an intravascular target using ultrasounds transmitted at the first and second frequencies to form an intravascular image. The first and second frequencies are between a xe2x88x923 dB low frequency and a xe2x88x923 dB high frequency of the transducer. The apparatus also includes a signal processing device and a computer-readable medium. The signal processing device is capable of being coupled co the transducer and to a display for displaying the intravascular image. Coupled to be read by the signal processing device, the computer-readable medium stores a computer-readable program for comparing a first intensity strength for echoes from ultrasound at the first frequency with a second intensity strength for echoes from ultrasound at the second frequency to detect blood speckle in the intravascular image.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.